1. Field of the Invention
This invention relates generally to operational amplifiers and, more particularly, to an operational amplifier having an improved slew rate/bandwidth characteristics.
2. Description of the Prior Art
One prior art operational amplifier utilizes inputs junction field effect transistors (JFETs) which directly drive a current mirror circuit. In such circuits, the bandwidth (BW) is related to the transconductance (g.sub.m) as follows: EQU BW=g.sub.m /C
where C is the Miller capacitance across the second stage of the amplifier.
The transconductance g.sub.m of a junction field effect transistor (JFET) may be expressed as follows: ##EQU1## where V.sub.po is the pinch-off voltage of the JFETs, I.sub.D is the JFET drain current, and I.sub.DSS represents the current that would flow through the JFETs with a zero gate-to-source voltage. Thus, ##EQU2## Since V.sub.po, C and I.sub.DSS are substantially constant in a given circuit, only I.sub.D can be varied to vary BW and g.sub.m, and both will vary as .sqroot.I.sub.D.
Slew rate, which is a measure of the rate of output change in response to an instantaneous input change is defined as EQU (dV.sub.OUT /dt)=I.sub.D /C
It is apparent that while BW is directly proportional to .sqroot.I.sub.D, slew rate is directly proportional to I.sub.D. Therefore, the values of BW and slew rate are fixed with respect to each other. That is, for a specific value of BW, there is a specific slew rate. Varying I.sub.D to cause an increase/decrease in BW will result in an increase/decrease in slew rate and vice versa.
This strict relationship can be varied by varying both I.sub.D and C. For example, if I.sub.D were increased four times and the capacitance C doubled, slew rate would double while bandwidth remains constant. However, capacitance C is large and occupies a large amount of die area. To increase C would require increased die size and a resulting increased cost.